JPS63164165A - Cadmium electrode for alkaline storage battery and its manufacture - Google Patents

Abstract

PURPOSE:To obtain a cadmium electrode to secure a good electron conductivity and to have almost no cycle deterioration in the gas absorption capacity, by using an active carbon fiber processed with fluorine contained resin. CONSTITUTION:This cadmium electrode is composed of an active substance layer which consists of an active carbon fiber with fluorine contained resin attached over the surface, and an active substance which consists of mainly cadmium oxide or cadmium hydroxide and a conductive core material on whose surface the active substance layer is formed. Since the water-repelling property of the fluorine contained resin is given to the surface of the active carbon fiber in such a composition, the hydrophobic property of the active carbon fiber is improved, and wetting to the alkaline solution can be controlled adequately. Moreover, since the surface of the active carbon fiber is protected from covering with the alkaline solution owing to the repeated charge and discharge, its catalystic property can be maintained for long. Cousequently, the gas absorption capacity of the cadmium electrode in which the active substance layer of dispersed active carbon fiber is formed is not deteriorated even though the charge and discharge cycle is repeated, and the original favorable condition can be maintained for a long period of time.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to improvements in cadmium electrodes used in alkaline storage batteries such as nickel-cadmium batteries, particularly in sealed batteries.

Conventional technology Conventionally, cadmium electrodes for this type of alkaline storage battery include:
Paste-type electrodes are common, in which a paste containing an active material, resin fibers, binder, etc. is applied to a conductive core material. This electrode has the advantage of being simpler to manufacture, cheaper to manufacture, and capable of providing a higher energy density than a sintered electrode, which requires a complicated manufacturing process. Therefore, as alkaline storage batteries have increased in capacity in recent years, this electrode manufacturing method has become mainstream.

However, paste-type electrodes have lower electronic conductivity than sintered electrodes in which a sintered substrate is impregnated with an active material, and they also have the disadvantage that the cadmium negative electrode absorbs oxygen gas generated from the positive electrode during overcharging. Ta.

Therefore, in order to improve the electron conductivity of the electrode and at the same time improve the oxygen gas absorption capacity of the electrode, activated carbon fiber (also called fibrous activated carbon), which is activated carbon fiber, is dispersed in the active material. It has been proposed to provide a layer of active material.

Problems to be Solved by the Invention However, in such a conventional structure, repeated charging and discharging cycles in an alkaline aqueous solution deteriorate the properties of the activated carbon fiber, and the gas absorption by the cadmium electrode, which is generally thought to be In the following two equations of the reaction, -02+H20+26-) 20H--------
-(1) The hydrophobicity of the activated carbon fibers involved in the electrochemical gas absorption reaction of equation (1) decreases, causing the three types of gas, liquid, and solid in the activated carbon fibers, which are considered to be the reaction sites of equation (1), to decrease. The phase interface became unstable, and the catalytic effect of formula (1) by the activated carbon fibers tended to decrease. Therefore, the gas absorption ability of the cadmium electrode deteriorates and the internal pressure of the battery increases, resulting in a short cycle life of the battery.

The present invention solves these problems and aims to prevent the deterioration of characteristics of activated carbon fibers due to charge/discharge cycles.

Means to Solve the Problem In order to solve this problem, the present invention provides an active material layer consisting of activated carbon fibers with a fluororesin attached to the surface and an active material mainly composed of cadmium oxide or cadmium hydroxide. and a conductive core material having the active material layer formed on its surface.

Function: With this configuration, the surface of the activated carbon fibers is given the water repellency of the 7-no-resin, so the hydrophobicity of the activated carbon fibers is improved, and wetting with an alkaline aqueous solution can be appropriately controlled. This configuration makes it possible to maintain the three-phase interface of gas, liquid, and solid, which is the reaction field of the above formula (1), stably for a long period of time.

In other words, the activated carbon fiber surface is prevented from being coated with an alkaline aqueous solution due to repeated charging and discharging, and its catalytic ability can be maintained for a long period of time. From this, the gas absorption ability of a cadmium electrode having an active material layer in which activated carbon fibers are dispersed does not deteriorate with the number of charge/discharge cycles, and maintains its initial favorable state over a long period of time.

Example Examples of the present invention will be described in detail below.

Activated carbon fibers having a fluororesin attached to their surfaces (hereinafter referred to as fluororesin-treated activated carbon fibers) were prepared as follows. Swt% of activated carbon fibers (6 to 16 μm in diameter) in a suspension of tetrafluoroethylene.

A acrylic activated carbon fiber having a length of 3 to 57 ff, an average pore diameter of 20 to 40 pores, a specific surface area of 600 to 120 oi/f was immersed and dried to obtain a fluororesin-treated activated carbon fiber.

To 100 parts by weight of cadmium oxide powder as the main active material, 0.6 parts by weight of the above-mentioned fluororesin-treated activated carbon fibers is added and mixed, and an appropriate amount of an ethylene glycol solution of polyvinyl alcohol is added and kneaded to form a paste. Make it. This paste was applied to a core material consisting of a nickel-plated perforated iron plate having a thickness of Q and 1 ff, and after drying, a chemical conversion treatment was performed by a known method to obtain an electrode plate with a thickness of about 0.5 fflJ+.

As a comparative example of the product of the present invention, a comparative electrode plate was obtained in the same manner as in the case of the product of the present invention, except that the fluororesin-treated activated carbon fibers were replaced with ordinary activated carbon fibers. The electrode according to the present invention and the comparative electrode were combined with a sintered nickel electrode using a known construction method to create a battery using the cadmium electrode according to the present invention and a battery B using the comparative electrode. This person used B's sealed nickel-cadmium alkaline storage battery (nominal capacity 500mAh, AA size) to compare the cycle changes in battery internal pressure during overcharging.

The figure shows a cycle of charging with a 0.1C current for 15 hours and discharging with a 0.20 current, and then charging only the 10th and 50th cycles in the middle with a 1C current for 2 hours.
The graph shows the change in internal pressure of the battery when discharging at a current of 0.2 G after the end of the charging. A-1 indicates the internal pressure of the battery according to the present invention during 1Q psi charge, and A-2 indicates the internal pressure during 50 cycles of charging. Similarly, B-1 and B-2 are the internal pressures of comparative battery B when charged for 10 cycles and 50 cycles.

As is clear from the figure, as a result of repeated charging and discharging cycles, the internal pressure of Comparative Battery B increased significantly, but the internal pressure of the battery of the present invention increased only slightly. In the figure, there is no difference between the pressures of batteries A and B during discharging and the atmospheric pressure (that is, the internal pressure value is close to 0 in the figure), and the pressure increase during overcharging is thought to be caused by oxygen gas. Therefore, the difference in pressure increase due to cycles between batteries A and B indicates that there is a difference in capacity between batteries A and B in cycle changes in oxygen gas absorption capacity. In other words, the difference in oxygen gas absorption capacity is that the properties of the activated carbon fibers contained in the active material layer of the battery's cadmium electrode do not deteriorate due to fluororesin treatment, and as a result, the gas absorption capacity of the battery's cadmium electrode is favorable. It is considered that it is maintained in its initial state.

Next, in order to examine the amount of fluororesin to be attached to the surface of activated carbon fibers, activated carbon fibers were prepared that were treated with fluororesin using ethylene tetrafluoride/suspension at four concentrations shown in the following table. Furthermore, using these four types and untreated activated carbon fiber, five types of batteries C, D, E,
Created F and G.

(Left below) These batteries were charged with a 1C current for 1.5 hours and subjected to a 1G current discharge cycle test for 1o cycles and 10
The table shows the results of investigating the maximum value of the battery internal pressure during 0-cycle charging and the discharge intermediate voltage during 6o psi discharge. From the table, Battery G without fluororesin treatment has a high discharge intermediate voltage of 1.2 V in 6 o cycles and good electronic conductivity, but 1
The maximum internal pressure of the battery at cycle 00 was very high at 15 kg/cl, and because no treatment was used, it is thought that the gas absorption capacity was significantly deteriorated compared to the other four types. On the other hand, high concentration (2o
wt%) In battery C treated with fluororesin suspension, the maximum internal pressure of the battery only increased slightly even after repeated cycles;
Although the gas absorption ability was maintained well, the discharge intermediate voltage was 1.05 V, lower than the others, and the voltage characteristics were poor. This is thought to be because the electronic conductivity of the electrode was reduced by treating the activated carbon fibers with a highly concentrated fluororesin suspension.

In contrast to these batteries G and C, battery F has improved gas absorption ability through fluororesin treatment, resulting in a lower maximum internal battery pressure during charging, and battery resin has improved electron conductivity due to lower treatment concentration. The discharge intermediate voltage is increased, and the battery characteristics of both F and D are improved. However, it is clear from the table that Battery E treated with Swt% fluororesin suspension is the best in both long-term maintenance of gas absorption capacity and electronic conductivity. Therefore, the amount of fluororesin in the activated carbon fiber is 15
It is considered that the amount that adheres when treated with a fluororesin suspension that does not exceed wt% is optimal in terms of battery characteristics.

Effects of the Invention As described above, according to the present invention, by using activated carbon fibers treated with fluororesin, a cadmium electrode can be obtained which ensures good electronic conductivity and has almost no cycle deterioration in gas absorption capacity. The effect is that a nickel-cadmium alkaline storage battery with excellent cycle life can be provided.

[Brief explanation of the drawing]

The figure is a diagram showing changes in battery internal pressure due to charging/discharging cycles of a battery of the present invention and a comparative battery.

Claims (2)

[Claims] (1) A cadmium electrode formed by supporting an active material layer mainly composed of cadmium oxide or cadmium hydroxide on a conductive core material, wherein the active material layer contains activated carbon fibers with a fluororesin attached to the surface. A cadmium electrode for an alkaline storage battery characterized by containing. (2) Activated carbon fibers with fluororesin attached are an alkaline storage battery characterized in that activated carbon fibers are immersed in a suspension with a fluororesin concentration of 2 to 15 wt% to be attached, and this is mixed into the active material. Manufacturing method of cadmium electrode for use.